Field of the Invention
The invention relates to the field of navigation systems based on the utilization of signals transmitted by several satellites of a so-called GNSS constellation, the acronym standing for “Global Navigation Satellite System”.
The invention relates more precisely to hybrid navigation systems based both on a satellite-based positioning system and on the information provided by an inertial unit.
The invention pertains to a system for excluding a failure of a satellite integrated within such a navigation system so as to exclude the measurements carried out on the signal transmitted by the satellite identified as having failed.
Description of the Related Art
A satellite-based positioning receiver provides information about the position and speed of the carrier by triangulation on the basis of the signals transmitted by flyby satellites visible from the carrier. The information provided may be momentarily unavailable since the receiver must have a minimum of four satellites of the positioning system in direct view in order to be able to take a fix. Said information is furthermore of variable precision, dependent on the geometry of the constellation on which the triangulation is based, and noisy since it is reliant on the reception of signals of very low levels originating from distant satellites having low transmission power. But said information does not suffer from long-term drift, the positions of the satellites flying in their orbits being known precisely over the long term. The noise and the errors may be related to the satellite systems, to the receiver or to the propagation of the signal between the satellite transmitter and the receiver of GNSS signals. Furthermore, the satellite data may be erroneous as a consequence of faults affecting the satellites. These spoiled data must then be flagged so as not to falsify the position arising from the GNSS receiver.
To preclude satellite faults and ensure the integrity of GNSS measurements, it is known to equip a satellite-based positioning receiver with a so-called RAIM (“Receiver Autonomous Integrity Monitoring”) precision and availability estimation system which is based on the geometry and the redundancy of the constellation of satellites used during triangulation and on the short-term foreseeable evolution of this geometry deduced from the knowledge of the trajectories of the satellites. However, the RAIM algorithm is used only for pure satellite location systems and not for hybrid location systems.
Furthermore, in the case where the geometry of the GNSS constellation exhibits certain particular configurations, the RAIM system allows exclusion of the defective satellite only once the positioning error has attained a high value, this not being compatible with a navigation system embedded on board a carrier which requires positioning information in real time with high precision.
Finally, in order to be able to operate, the RAIM algorithm requires at least 6 satellites in visibility of the receiver, this not necessarily being the case.